1. Technical Field
The present invention relates to a variable gain power amplifier.
2. Related Art
In the field of communication apparatus employed for the Industry Science Medical (hereinafter, ISM) band or specific low-power radio transmission, the increasing demand for a smaller size, lower power consumption and higher accuracy in transmission frequency has been expanding the employment of ICs for performing various functions. Transmitters used in those communication apparatus have similarly come to include in an IC the functional circuits such as an oscillator, a PLL frequency synthesizer, a modulator, a power amplifier, and a control microcomputer.
Regarding the transmitter mainly used for the purpose of remote keyless entry (hereinafter, RKE) in particular, the circuits definitely have to be packaged in an IC, in order to be disposed in an automobile key. To be more detailed, the necessary circuits have to be implemented in a small package with a minimum necessary number of functional pins, thus to be coupled with a limited number of simple external parts. Also, for achieving compatibility with both domestic and overseas standards, it is desirable that a single IC is capable of performing the Amplitude Shift Keying (hereinafter, ASK) or On-Off-Keying (hereinafter, OOK, including enabling action based on an on/off control) and the Frequency Shift Keying (FSK), and that the output power is micro-adjustable.
Conventional power amplifiers include those disclosed in Japanese Laid-open patent publications No. H06-164249 and No. 2000-295050. FIG. 5 is a circuit diagram of a variable gain power amplifier disclosed in the former publication. A transistor 101 constitutes a power amplifying unit self-biased by resistances 102, 103. To the base of the transistor 101, a variable current source 110 is connected. The variable current source 110 is set up as a current mirror circuit including transistors 111, 112. In this amplifier, an input voltage for controlling the current mirror is applied to a terminal 104, to control the gain through a shunt control of the base bias current.
FIG. 6 is a graph showing a gain control characteristic of the variable gain power amplifier of FIG. 5. The horizontal axis represents the input voltage (V) for current mirror control, and the vertical axis represents the gain (dB), respectively. As is understood from the graph, the variable current source 110 is applied to the base of the power amplifying transistor 101, so that the gain control is performed through controlling the base current depending on the control voltage.